One method of fabricating chips with suspended micromachined microstructures is generally termed bulk-micromachining. In bulk-micromachining, a block of material is subtractively etched to remove material leaving behind the desired microstructured shape suspended from an unremoved portion of the substrate. Accordingly, in bulk-micromachining, the microstructure and the supporting portion of the substrate are of the same material.
Another method of fabricating chips with suspended micromachined microstructures is generally termed surface-micromachining. Surface-micromachining involves forming the microstructure over a sacrificial layer upon a substrate and then removing the sacrificial layer. For instance, a sacrificial oxide spacer layer such as silicon dioxide is deposited over the surface of a substrate of a wafer or die. The sacrificial spacer layer is selectively etched to open up a number of holes in the spacer layer down to the substrate, in which anchors for supporting the microstructure will be formed. A thin film microstructure material, such as polysilicon, is deposited over the sacrificial layer. In the holes where the sacrificial layer had been etched down to the substrate, the microstructure material contacts the substrate to form anchors for supporting the microstructure. Enough microstructure material is deposited to completely fill the holes as well as to form a uniform layer over the sacrificial layer. The microstructure material is then patterned into the desired shape. Finally, the sacrificial layer is removed (i.e., sacrificed) by, for instance, wet etching, thus leaving behind a microstructure suspended above the substrate by the anchors. U.S. patent application Ser. No. 08/028,922 filed on Mar. 4, 1993 entitled Monolithic Chip Containing Integrated Circuitry and Suspended Microstructure and assigned to the same assignee as the present application discloses in detail one such method for manufacturing a suspended microstructure.
Suspended microstructures are commonly used as sensors, such as acceleration sensors. U.S. patent application Ser. No. 08/028,922 discloses one such suspended microstructure accelerometer. The fingers of the suspended microstructure are interleaved with adjacent stationary fingers. A voltage is applied between the suspended microstructure and the stationary fingers so as to form a capacitor.
The suspension of a microstructure typically is made very thin and/or narrow so that it is resilient and will tend to deform under acceleration or other force. The deformation under force causes the suspended microstructure to move relative to the stationary fingers thus causing a change in voltage across the capacitor formed by the microstructure and the stationary fingers. The change in voltage is sensed and the force can be determined from the sensed change in voltage. Accordingly, the microstructures are typically extremely fragile such that the fabrication of wafers embodying the suspended microstructure is difficult and usually produces a relatively low yield of acceptable dies. A relatively large number of microstructures either break during fabrication or are bent enough so as to cause the suspended portion of the microstructure to contact the substrate or another portion of itself. The microstructure tends to stick to anything it comes in contact with, thus rendering the microstructure useless.
Liquid surface tension effects are among the most significant causes of microstructure breakage or failure because such surface tension tends to draw the microstructure surfaces towards other surfaces. Liquid surface tension effects occur, for instance, during drying after a wet etching fabrication step.
In fabricating suspended surface-micromachined microstructures, the sacrificial layer is commonly removed by a wet etching process in which the wafer is exposed to a chemical etching solution which dissolves the sacrificial layer but does not affect the material from which the microstructure is formed. The wafer is then washed in a rinse liquid. As the rinse liquid is removed, the surface tension of the liquid exerts forces on the delicately suspended microstructure, tending to pull the surface of the microstructure into contact with the substrate or with other nearby surfaces of the microstructure. The combination of various forces, including adhesive forces and electrostatic forces, makes it extremely difficult to separate the microstructure and/or substrate surfaces once they have come in contact. Electrostatic forces may also contribute to the initial attraction of the microstructure surfaces to other surfaces, leading to the initial contact. Accordingly, when undesirable contact occurs, the die is typically irreparable and must be discarded.
Dry etching to remove the sacrificial layer might eliminate the surface tension problem but is typically not practical because a dry etch process would likely damage the suspended microstructure material, e.g., polysilicon, due to the low selectivity of such etching techniques. Also, dry etching would not eliminate electrostatic charges, which may also be a cause of sticking.
Several researchers have reported on this problem and proposed solutions. For instance, H. Guckel, J. J. Sniegowski, T. R. Christenson and F. Raissi, "The Application of Fine-Grained Tensiled Polysilicon to Mechanically Resonant Transducers", Sensors and Actuators, A21, (1990) pp. 346-351, suggest a method in which the final rinse liquid is frozen and sublimated to avoid the harmful effects of liquid surface tension on microstructures. The disclosed method describes the transfer of wet wafers into a refrigeration unit to freeze the liquid (a water/methanol mixture). The wafer is then placed in a vacuum system to sublimate the frozen fluid. The technique is not practical because it requires the direct transfer of wet wafers into refrigeration systems and vacuum systems. Further, sublimation of the frozen fluid requires very long times, on the order of hours.
More recent approaches attempt to provide a temporary support structure to the microstructure during fabrication, which support structure is eliminated during the final steps of fabrication, after all possibly harmful fabrication steps have been completed. U.S. patent application Ser. No. 07/872,037 filed on Apr. 22, 1992 and assigned to the same assignee as the present application discloses a method in which, after the microstructure layer is patterned (exposing some of the sacrificial layer), but before the underlying sacrificial layer is removed, a photoresist layer is deposited and masked to expose only selected portions of the sacrificial layer which is exposed after the microstructure patterning. The selected portions of the spacer layer are then etched down to the substrate. Since most etching processes can remove material a few microns beyond the edge of the mask, a few microns of spacer can be removed from underneath the edges of the suspended microstructure even though those parts of the sacrificial layer are not exposed, but are covered by the microstructure. Another photoresist layer is then deposited over the wafer which fills in the holes which were just etched into the spacer layer, including the portions which extend underneath the microstructure. The photoresist is then developed away except for the photoresist that has filled in under the edges of the microstructure which is not developed away since it is occluded by the microstructure from the imaging process. These photoresist "pedestals" remain and will support the microstructure after the sacrificial layer is removed.
In addition, the photoresist exposure mask used for this step may be designed to leave some photoresist bridges in the layer defined by the microstructure for bridging gaps between otherwise non-contacting portions of the microstructure. The sacrificial layer is then removed in a wet etching process which does not affect the photoresist or microstructure material, both of which remain after the etching process. The photoresist pedestals which were formed beneath the edges of the microstructure remain and vertically support the microstructure above the substrate while the photoresist bridges which remain provide lateral support tending to prevent the microstructure or at least a portion of it from laterally bending. As close to the end of the fabrication process as possible, the photoresist pedestals and bridges are removed by a long oxygen plasma stripping process which does not present any liquid surface tension problems, thus leaving the microstructure freely suspended over the substrate in its final form.
The above described method is effective in increasing yield; however, it requires two additional masking procedures, including all related steps. Further, there is still a chance of sticking and failure of the microstructure during the final dry etching release step in which the photoresist pedestals and bridges are removed. Although there is no liquid surface tension problem during dry etching, the electrostatic charge problem discussed above remains and may still cause failure due to sticking of the microstructure.
G. K. Fedder, J. C. Chang, and R. T. Howe, "Thermal Assembly of Polysilicon Microstructures With Narrow Gap Electrostatic Comb Drive", Technical Digest, 1992 IEEE Solid-State Sensor and Actuator Workshop, Hilton Head Island, S.C., page 63, discloses another method for forming temporary support for the microstructure. This paper discloses a method by which, after the microstructure layer is deposited and patterned, a substantially thinner layer of the same material is deposited. The thin layer is etched to leave bridges between adjacent, non-contacting surfaces of the microstructure thus providing support and strength in the lateral direction. The final step in releasing the microstructure consists of melting the bridges by injecting through them an electric current which is sufficient to blow the thin links but which will not damage the thicker microstructure.
This approach allows the microstructure to be released after wafer fabrication or even after packaging. However, since the bridges must be thinner than the microstructure, this approach requires additional deposition, masking and etching steps. Further, extra probing/bonding pads, silicon area and leads are needed for inputting the current for melting the links if the final microstructure release is to occur after packaging.